A. Field
The present invention relates to a reactor and a process for solid phase continuous polymerisation of polyethylene terephthalate (PET).
B. Related Art
More exactly, the present invention relates to a reactor and a process for solid phase continuous polymerisation of polyethylene terephthalate (PET) in order to achieve an increase of the intrinsic viscosity (I.V.) of a low molecular weight PET pre-polymers flow.
Generally, PET manufacturing process includes four steps: esterification, pre-condensation, finishing step and solid phase polymerisation.
In the past, PET manufacturing process was carried out by transesterification of dimethyl terephthalate (DMT) with ethylene glycol (EG) while direct esterification of purified terephthalic acid (PTA) with ethylene glycol (EG) is nowadays well-established, whose advantages mainly reside both in its cost effectiveness and in its higher reaction rate.
Ethylene glycol (EG) and oligomers are present among the by-products at the end of the reaction.
Ethylene glycol (EG) is usually recycled in feed at the beginning of the process, thus increasing the overall yield; the oligomers have to be removed and then be disposed of or, alternatively, recycled to continue increasing their polymerisation grade.
It is known that the oligomers in the field of interest, i.e. within the PET manufacturing processes and, more generally, within the polyester ones, are normally molecules having 2, 3 or maximum 4 polymerisation grade, at gaseous state at high temperature conditions and at liquid state at temperatures lower than 120÷150° C.
Said oligomers are present at gaseous state at the end of the reaction, but tend to condense on the colder metal surfaces of piping/fittings they pass through; once condensed, they tend to produce sticky mixture together with finest dusts that are not removed by cyclone separation systems usually provided downstream the PET manufacturing plants.
Such sticky mixture settles onto the pipes walls and progressively reduces the flow areas thereof, until causing the whole system shutdown in case it is not properly removed during maintenance operations.
In consequence of what above described, it becomes evident that the oligomers removal from the gaseous flow exiting the plant is essential.
The conventional melt-phase polymerisation processes routinely use the first three steps previously mentioned, i.e. esterification, pre-condensation and finishing step, to produce so-called pre-polymers for the subsequent solid phase polymerisation.
Solid phase polymerisation is a thermal treatment process that allows to increase the molecular weight of a polymer at different levels, depending on the kind of final product that it is wanted to be obtained.
It is known that the molecular weight of a polymer can be measured by the measure of its intrinsic viscosity I.V. (Intrinsic Viscosity).
It is also known that the increase of the molecular weight of a polymer can be achieved by subjecting low molecular weight polymers (pre-polymers), preferably under granular form, to a solid phase continuous polymerisation step on fluidised bed. Such process further allows to provide polymers with low acetaldehyde content, content that must be lower than 1 p.p.m. for the manufacturing of PET bottles intended for food use.
The pre-polymers fed to the solid phase polymerisation step can be under the form of completely amorphous pellets or partially crystallised pastilles.
Amorphous PET pellets are thermally stable only up to the glass transition temperature, which is about 80° C.; however, the temperature of the solid phase polymerisation process is higher than 200° C.
By subjecting amorphous PET pellets to a heating phase as well as to a temperature maintaining phase above the glass transition temperature, a progressive crystallisation of the polymeric bulk is achieved, by hardening the polymer matrix inside pellets themselves, so achieving thermal stability up to temperature values close to the onset (about 235° C.) of melting point.
Due to this reason, pre-crystallisers and crystallisers are generally necessary plant units upstream the conventional solid phase polymerisation reactor.
The purpose of the crystallisation step prior to solid phase polymerisation is to prevent the agglomeration of the granules during the polymerisation process, especially at the highest temperatures.
As a matter of fact, it is known that in nowadays employed industrial solid phase polymerisation plants, the granules agglomeration phenomenon frequently happens; this problem is particularly evident during PET solid phase continuous moving bed polymerisation for producing bottles intended for food use, when polymerisation is carried out at temperatures above the amorphous PET pre-polymer glass transition temperature, but below the melting point.
According to the conventional processes today available and employed for solid phase polymerisation, the PET pre-polymer (crystallised or partially crystallised) is fed at the top of a vertical moving or static bed reactor for solid phase polymerisation, in which the pre-polymer moves downwards by gravity in contact with an inert gas stream.
According to known prior art, the inert gas primarily functions to remove unwanted by-products, in particular ethylene glycol, acetaldehyde and water vapour, which are generated during polymerisation, while PET gradually moves towards the bottom of the vertical reactor.
In general, there are three important requisites that are to be met for correct operation of a continuous solid phase polymerisation process.
First, a steady and uninterrupted flow of PET granules must be maintained.
As a consequence, it is highly important that agglomeration of PET granules is avoided, since it would impede the smooth flow of granules and it would make difficult the discharge of the product from the reactor, thereby causing the plant control losing.
Secondly, a suitable combination of residence time and temperature of granules in the reactor is required to achieve the desired molecular weight, which is measurable, as indicated above, in terms of intrinsic viscosity I.V.
Since reaction rate increases with temperature increase, and I.V. increases with residence time increase in the reactor, desired I.V. can be attained either by using a relatively long residence time, with a relatively low temperature, or a relatively short residence time with a relatively high temperature.
However, the ideal combination of residence time and temperature must be chosen taking into account the first of the requisites indicated above, i.e. the need to maintain a linear flow, thereby avoiding the granules agglomeration phenomenon.
Third, the flow regime of PET granules processed inside the solid phase polymerisation reactor, must be as close as possible to the ideal plug-flow behaviour; this way all PET granules passing through the reactor undergo the same conditions of treatment and a narrow molecular weight distribution in the obtained product and, more in general, a narrow distribution of polymerised granules final attributes, which is a key factor for the required performance during subsequent high I.V. PET product processing/converting steps, will be achieved.
As regards the first requisite, that is the need to avoid the PET granules agglomeration, it is to be said that this phenomenon is mainly affected by temperature, granules size, bed height, granules flow velocity within the reactor and PET morphology.
PET granules, initially moving freely in a moving bed can stick and clot if, for instance, temperature or bed height increases or if cross-sectional velocity decreases.
At solid phase polymerisation conditions, PET is only partially crystallised and, as a consequence, such a PET is not a rigid body, but it is rather a slightly sticky body.
Since the PET tendency to become sticky increases with temperature increase, the agglomeration tendency also increases with temperature increase.
A fixed bed of PET granules held motionless inside a solid phase polymerisation vertical, cylindrical reactor is taken into account.
Under these conditions, at polymerisation temperature and under pressure due to the weight of the PET granules bed, granules to be polymerised creep into one another at contact points and, in time, polymer granules will tend to agglomerate and form larger lumps.
The most effective way to prevent lumping is to constantly renew the intergranular contact areas, so that polymer granules do not have a chance to creep into one another.
This purpose is achieved by maintaining a constant flow of polymer granules at sufficiently high velocity.
Since agglomeration tendency increases with the increase of the specific surface area (area per unit mass) or, more precisely, with the increase of the specific contact area of polymer granules, it also increases with the decrease of the size of polymer granules.
A reduced granules size contributes to accelerate the polymerisation process, however, on the other hand, the agglomeration tendency of polymer granules increases.
In presence of small size granules it is therefore required to counterbalance the increased agglomeration tendency with a temperature decrease, which, on the other hand, brings the final values of the process rate back to the typical ones for larger size granules treated at a higher temperature.
Furthermore, if the particle size is reduced below certain limits, agglomeration occurs practically at any temperature.
In a static or moving bed situation, the compaction pressure undergone by polymer granules is approximately proportional to the weight of the polymer granules inside the bed which, in its turn, is proportional to the bed height above the granules; therefore, polymer agglomeration tendency is highest at the bottom of the bed and lowest at the top.
From what above mentioned it results that polymer lumps start generally to form near the bottom of the bed; for this reason there is a practical limit on the bed height of a solid phase polymerisation reactor.
At a sufficiently high flow velocity, polymer granules change their positions relative to each other (by sliding, for instance) and lumps formation is thereby prevented.
Since the change rate of polymer granules contact areas and the reduction of bed bulk density increase with the increase of the granules velocity, polymer agglomeration tendency inside the reactor decreases with the increase of the granules cross-sectional velocity.
For each combination of reactor temperature, bed height, and particle size, there exists a minimum granules velocity necessary to prevent agglomeration.
For each given size and shape of polymer granules, the minimum velocity to prevent agglomeration increases with the increase of temperature and bed height.
In case polymerisation temperature increases or in case bed height increases, a higher velocity has to be used; in case, for instance, of commercial scale vertical reactors, with outputs up to 300 metric tons per day and which are conventionally 18 to 22 meters high, a velocity of at least 2 meters per hour is generally required.
A well designed solid phase polymerisation commercial scale plant must be capable of continuously producing outputs having intrinsic viscosity I.V. in compliance with the required specification at a sufficiently high throughput.
The currently used plants employ single or multiple vertical cylindrical reactors 10 to 30 meters high; in those plants the reactor is operated at a temperature of 200 to 230° C. and at a granules moving velocity of 1.00 to 2.52 meters per hour. Within the above-mentioned ranges of temperature, bed height, and granules velocity, the choice to achieve a product with the desired I.V. can be made.
These conventional currently used plants allow to produce PET having an I.V. of 0.72 to 0.86 dl/g, using PET pre-polymers with an I.V. of 0.55 to 0.65 dl/g; these conventional plants can increase polymer I.V. by about 0.12-0.25 dl/g.
Amorphous PET granules have intrinsic viscosity values generally comprised in the range 0.57÷0.62 dl/g; the reaction time necessary to achieve final I.V. value in the range 0.72÷0.85 dl/g, required for the most bottle manufacturing applications, is of 12÷18 hours.
Usually, PET I.V. is brought to the above-mentioned values, commercially required for bottle manufacturing, through polymerisation processes carried out in continue solid phase vertical reactors, in which solid bed of PET granules moves downwards by gravity.
For some specific applications, for instance PET pre-polymers polymerisation for standard bottle manufacturing, characterised by initial I.V. values of 0.25-0.45 dl/g, it is necessary to increase said I.V. by more than 0.25 dl/g.
This result is hardly achievable and often it is not achievable in a conventional plant using said vertical reactors.
In a conventional process, there are two ways to raise the product I.V.; namely, to increase the reactor temperature or to increase the granules residence time inside the reactor.
The residence time inside the reactor is constrained by bed height and by granules velocity; it can be increased either by increasing bed height or by decreasing granules velocity.
Increasing the reactor diameter allows an increase in the throughput rate, but not in residence time at constant granules velocity.
On the other hand, if reactor temperature is raised to increase the final product I.V., polymer agglomeration tendency will consequently increase.
To prevent polymer agglomeration, bed height has to be decreased or granules velocity has to be increased. However, both these modifications reduce residence time inside the reactor and nullify the effect of the temperature increase.
Alternatively, if the residence time inside the reactor is increased either by increasing the bed height (assuming the reactor is sufficiently tall) or by reducing the granules velocity, an increase of the polymer agglomeration tendency is caused.
To prevent the agglomeration phenomenon, the reactor temperature must be lowered and this once again nullifies the effect of the residence time increase on the product final I.V.
These constraints limit the ability of conventional plants, which use single or multiple vertical reactors, to increase the polymer intrinsic viscosity I.V.
A similar situation is encountered when a commercial scale plant with a capacity above 360 metric tons per day has to be designed for conventional continuous solid phase polymerisation processes.
In fact, in a conventional process, there are two ways to reach high plant production capacity: again by increasing the reactor temperature or by increasing the product volume (“hold-up”) in the reactor.
As far as drawbacks due to the temperature increase are concerned, the same above described issues have to be considered.
On the other hand, the product volume (“hold-up”) of PET granules in the reactor is constrained by bed height, reactor diameter and granules velocity.
If the product volume (“hold-up”) is increased either by increasing bed height or reactor diameter, or by decreasing granules velocity, polymer agglomeration tendency will increase too.
Thus, these constraints limit the maximum capacity of conventional solid phase polymerisation processes, which. use one or more vertical cylindrical reactors.
Nowadays, growing PET demand has given rise to a need for solid phase polymerisation processes by means of which it is possible to achieve a higher increase of PET molecular weight and a higher production capacity, typically higher than 300 metric tons per day on single plant.
One of the drawbacks of the plants according to the known prior art is due to the considerable vertical size of the plant structure due to the presence of the pre-crystallisation unit, which is stacked onto the crystallisation unit, stacked in its turn onto the solid phase polymerisation reactor.
A second drawback is represented by the long residence time, inside the conventional reactor, required to achieve the desired I.V. increase starting from amorphous pre-polymers; for example, to get from an I.V. of 0.60 dl/g to that of the final product, which is equal to 0.80 dl/g in the case of application in the manufacturing of standard bottles intended for food use, an average residence time equal to 15 hours is needed.
Such drawback is substantially due to two reasons: the fact that the solid phase polymerisation kinetic is linked to the reaction gaseous products diffusivity from the granules core to their outside and the fact that the pre-crystallisation and crystallisation steps, which are mandatory to secure enough flowability to the granules in a conventional moving bed solid phase polymerisation reactor, limit the polymeric matrix portion able to react, being the only non-crystalline portion to be involved in the reaction. A known solution to the above-mentioned problems consists in the use of PET sand of spherical granules having a sufficiently small size (typically 100÷200 μm) and of a great amount of inert gas in the reactor.
Thanks to these expedients the pre-crystallisation and crystallisation steps have been avoided, the reaction time has been reduced and the solid phase polymerisation reactor acquires lower size.
The aforesaid solution is nevertheless not optimal from the point of view of cost, of reactor size, of the achievable I.V., of I.V. distribution and of the reaction time.